PREPARATIONS FOR PARTIALVENEER CROWNSPresented by:-Himanshu khatriB.D.S. Final yearDepartment OfProsthodontics
CONTENTINDICATIONSCONTRAINDICATIONSADVANTAGESDISADVANTAGESARMAMENTARIUMPOSTERIOR PARTIAL VENEER CROWN PREPARATION MAXILLARY PREMOLAR THREE QUARTER CROWNS MAXILLARY MOLAR THREE QUARTER CROWN MAXILLARY MOLAR SEVEN-EIGHTHS CROWN MANDIBULAR PREMOLAR MODIFIED THREE QUARTER CROWNANTERIOR PARTIAL VENEER CROWN PREPARATION MAXILLARY CANINE THREE QUARTER CROWN PINLEDGE PREPERATION ON MAXILLARY CENTRAL INCISOR
INDICATIONS1) Intact buccal wall & wellsupported by sound toothstructure.2) Teeth with crown length that isaverage or exceeds average.3) Teeth with normal anatomiccrown form, i.e. withoutcervical constriction.4) Anterior teeth with adequatelabio-lingual thickness.
CONTRAINDICATIONS1) High caries rate2) In short clinical crown becauseretention may not be adequate.3) In endodontically treatedteeth or non-vital teeth4) In teeth that are proximallybulbous5) Deep cervical abrasion6) Teeth with extensive corerestorations7) Bell shaped teeth (severe
ADVANTAGES1) Conserves tooth structure.2) Easy access to supra-gingivalmargins for finishing.3) Less gingival involvement thanwith complete cast crown.4) Aesthetics are superior to thatof the complete crowns.5) Complete seating of therestoration can be easilyverified during cementation.
DISADVANTAGES1) Retention is less than that ofcomplete veneer crown2) Skilful preparation is crucial toavoid metal display3) Preparation is limited to teethwith normally shaped, averagelength clinical crown.4) It is not indicated for use incases of non vital teeth
Armamentarium• Narrow(0.8 mm) round-tipped tapered diamond(regular or coarse grit)• Regular size(1.2mm)rounded-tipped taperedcarbide• Wheel shaped diamond bur• Tapered and straight carbide fissure burs• Small round carbide bur• Small-diameter twistdrill• Inverted-cone carbide bur• Finishing stones• Mirror• Explorer and periodontal probe• chisels
Occlusalreduction (i)Before any partial veneer crown preparation , mark theproposed location of the margin of the completed preparationon the tooth with a pencil.Mark location with pencil
(ii)Place depth grooves for the occlusal reduction. These shouldbe made with a tapered carbide or narrow diamond in thedevelopmental grooves of the mesial and distal fossae and onthe crest of the triangular ridge. In the central groove theyshould be slightly less (about 0.2 mm) than 1 mm deep toallow for finishing; on the centric (lingual) cusp they should beslightly less than 1.5 mm deep in the location of the occlusalcontacts.Minimum clearances for reduction
(iii)Place three depth grooves on the lingual incline of the buccal cusp.Initially, these should be kept somewhat shallow as they approachthe buccal cusp ridge. In the area of occlusal contact, the grooveshould be about 0.8 mm deep so that there will be at least 1 mm ofclearance after finishing.Additional guiding grooves placed(on the lingual cusp)
(iv)Verify groove depth with a periodontal probe. When this is found tobe acceptable, remove the islands of tooth structure remainingbetween the groovesOcclusal reduction
(v)Assess the amount of occlusal clearance in the intercuspal positionand in all excursive movements of the mandible.Common error is insufficient reduction
(vi)Grinding a small concavity on the incline of the buccal cusp mayhelp obtain sufficient clearance while maintaining the originalocclusocervical dimension of the buccal tooth surface.Occlusocervical dimensionsInterproximal clearance relative to adjacent teeth
Axial reduction(vii)Place grooves for axial alignment in the center of thelingual surface and in the mesiolingual and distolingualtransitional line angles. These should be parallel to thelong axis of the tooth and should not exceed half thewidth of the tip of the diamond used to place them.Because the path of withdrawal of a partial veneer iscritical, assess these grooves carefully when correction isstill possible.A common error is to incline the path of withdrawaltoward the buccal. This either reduces retention or leadsto an excessive display of metal.
After verifying the alignment, remove tooth structure between the guidegrooves (with a smooth continuous motion) and place a cervicalchamfer.Carry the diamond into the proximal embrasure and reduce the proximalwall.(viii)A periodontal probe placed in each groove should be carefully viewed inboth planes (mesiodistal and buccolingual). It often helps to pour anirreversible hydrocolloid (alginate) impression in fast-setting plaster and toevaluate the cast with a dental surveyor, particularly if multiple partialveneers are being used as retainers for an FPD.Axial reduction
(ix)After verifying the alignment, remove tooth structure between the guidegrooves (with a smooth continuous motion) and place a cervicalchamfer.Carry the diamond into the proximal embrasure and reduce the proximalwall.For proper reduction of the axial tooth surface, it is important tounderstand the factors that determine correct positioning of the proximalgroove. A proximal groove is placed parallel to the path of withdrawal.Normally, unsupported tooth structure will remain on the buccal side ofthe groove, and this side is flared to remove it. illustrates the relationshipamong the initial axial reduction, groove placement, and location of thecavosurface angle where the flare meets the intact buccal wall. Thecavosurface angle is especially significant when preparing a tooth for apartial veneer that should display a minimum of metal; the further to thebuccal the marginis, the more gold will be visible.Groove is placed perpendicular to the preparedsurface
(x)A subtle but extremely important variable that determines the final location ofthe cavosurface angle is the apical extension of the preparation. As thecervical chamfer extends closer to the cemento-enamel junction, more axialtooth structure is removed. Consequently, the deepest portion of the groove(its pulpal wall) will be located slightly closer to the center of the tooth.This results in a flare that can extend farther onto the facial or buccal surfacethan desirable. Marking the location of the intended facial flare on the toothwith a pencil before initiating the proximoaxial reduction is helpful. Theintersection of this mark with the reduced occlusal surface is a convenientreference point.Distal proximal reduction is stopped prior to breakingproximal contact
Stop the proximal reduction well short of the pencil mark and usuallyslightly short of breaking the proximal contact .The resulting flange should be parallel to the linguoaxial preparation, withthe chamfer placed sufficiently cervical to provide at least 0.6 mm ofclearance with the adjacent tooth and the axial wall allowing for a proximalgroove of at least 4 mm of length occlusocervically.Buccal wall are flared to leave nounsupported enamel
Groove placement(xi)Preparation of the proximal grooves is best done with a tapered carbide bur.Position the bur against the interproximal flange parallel to the path ofwithdrawal and make a groove perpendicular to the axial surface. Thegroove need not be deeper than 1 mm at its cervical end but may be deepernear its occlusal end.During this stage, the bur must be held precisely parallel to the selectedpath of withdrawal.Floor of the groove should be flatand smooth, chamfer extendcervicallyOcclusocervical height for aproximal groove is 4 mm
The criteria that need to be met consist of the following:-The grooves should resist lingual displacement of a periodontal probe orexplorer.The walls of the grooves should not be undercut relative to the selectedpath of withdrawal.The walls should be flared toward the intact buccal surface of the toothAllowing it to tip axially will result in excessive taper between opposingproximal grooves, which is a common error.Buccal aspect of the groove has beenadequately flared
Depending on available access, it may be feasible to complete the flaringwith the same rotary instrument that was used to place the groove.However, removing the last lip of unsupported tooth structure with a chiselis often a better option, because this minimizes the risk of damage to theadjacent tooth.Initial preparation of the mesialgrooveInitial flaring Remove allunsupported enamel
Buccalocclusal contrabevel(xii)Connect the mesial and distal flares with a narrow contrabevel that followsthe buccal cusp ridges. This can be placed with a diamond, a carbide, oreven a hand instrument. Its primary purpose is to remove any unsupportedenamel and thereby protect the buccal cusp tip from chipping duringfunction.The bevel should remain within the curvature of the cusp tip rather thanextend onto the buccal wall. This will result in a convex shape of therestoration, and light will be prevented from reflecting back to a casualobserver. Thus the restoration will be less obvious, and the outline form ofremaining buccal enamel will be perceived as the shape of the tooth.Buccocclusal contrabevel remains within the curvature of thecusp tip
Occlusal offsetIf additional bulk is needed to ensure rigidity of the restoration, it can beprovided with an occlusal offset. This V-shaped groove extends from theproximal grooves along the buccal cusp. It is not usually necessary forposterior partial veneer crowns but is essential for the structural durability ofanterior partial veneer crowns.
FinishingRound all sharp internal line angles to facilitate subsequent procedures.A fine-grit diamond or carbide can be used to blend the surfaces.Re-evaluate the flares, paying particular attention to any remainingundercuts, which must be removed.The flares should be straight and smooth, with sufficient clearancebetween them and the adjacent tooth.A minimum clearance of 0.6 mm is recommended.The mesial flare cannot extend beyond the transitional line angle.However, because the distal margin is less visible, it may extend slightlyfarther to the buccal, allowing better access for oral hygiene.
THREE QUARTER CROWN FORMAXILLARY MOLARThe principles used in a premolar preparation also apply for a maxillarymolar.However, some additional leeway may exist for groove placementbecause more tooth structure is present on molars than on premolars.Also, because of their less prominent position in the dental arch, molarsare less visible.As a result, the mesioproximal flare can sometimes be extended onto thebuccal surface without incurring esthetic liability.
MAXILLARY MOLAR SEVEN-EIGHTHS CROWNSThe seven-eighths crown preparation includes, in additionto the surfaces covered by the three-quarter crown, thedistal half of the buccal surface. Therefore the mesialaspect of this preparation resembles that for a three-quarter crown; the distal aspect resembles that for acomplete crownA. Occlusal reductionB. Axial reductionC. Groove placement, flaring, contrabevel
The mesial half of the buccal tooth surface remains intact and is protectedby a narrow contrabevel or chamfer similar to the one used in the three-quarter crown preparation.A distal groove may be placed, although generally this is not necessary. Agroove in the middle of the buccal surface is placed parallel to the path ofwithdrawal. Distal to this groove the buccal surface is reduced in twoplanes, cervical and occlusal, with the cervical paralleling the path ofwithdrawal and the occlusal following the normal anatomic contour.The lingual surface of the tooth also is reduced in two planes, and centriccusp bevels are incorporated.
Occlusal reductionUpon completion of the occlusal reduction, adequate clearance should existin all excursive movements of the mandible. Minimum measurements arethe same as for the three-quarter crown preparation.Place depth grooves in the central and developmental grooves as well ason the crests of the triangular ridges. To delineate the extent of the lingualcentric cusp bevel, they should extend onto the lingual surface of the tooth.On the lingual incline of the mesiobuccal cusp they will resemble depth cutsfor the three quarter crown preparation. On the distobuccal cusp theyshould be approximately 0.8 mm deep to provide sufficient occlusalclearance for this non-centric cusp.Occlusal depth grooves
Remove the tooth structure between the depth grooves. Concave shapingof the resulting mesiobuccal incline may again prove useful because it willpermit the occlusocervical height of the cusp to be maintained. Whencompleted, this bevel should provide 1.5 mm of clearance in the intercuspalposition as well as throughout all excursive movements of the mandible.Mesial half of occlusal reductioncompletedOcclusal reduction completed
Axial reductionPlace three alignment grooves in the lingual wall and transfer the selectedpath of withdrawal to the distobuccal transitional line angle area, where afourth alignment groove can be placed.Start the reduction in the middle of the lingual surface. The mesial half isprepared like a three-quarter crown and the distal half like a completecrown.Carry the facial reduction sufficiently mesial to include the buccal groove.Although the occlusal half of the buccal surface of maxillary molars israther flat, some additional reduction may be necessary in the occlusalthird. This follows the normal anatomic configuration of the tooth and oftenresembles a small version of the centric cusp bevel. If correctlyperformed, the reduction will allow for contouring of the restoration so thatwhen viewed from the mesial, the distal half of the restoration is hiddenbehind the mesiobuccal cusp. A frequent error is to over taper the buccalwall segment, with resulting loss of retention.Parallel guiding grooveplaced in the lingual toothsurface
Groove placement, flaring,contrabevelPrepare the mesial groove like the three-quarter crow.Place the buccal groove parallel to the mesial groove and perpendicular tothe buccoaxial wall. Often it is not necessary to flare the buccal groovebecause the flat configuration of this area of the tooth precludes anyunsupported enamel after the groove is placed. The buccal groove shouldresist mesiodistal displacement of a probe.Proximal groove placed Buccal groove with flaring of the mesialgroove
Connect the two grooves with a smooth contrabevel that follows the ridgeof the mesiobuccal cusp.This bevel should meet the same criteria as described in the three-quartercrown preparation. Adequate clearance must be established interproximallyupon completion. All surfaces are finished to the same specifications as thepreceding preparations.Mesial groove is smooth and has a 90-degree cavosurfaceangleSeven-eight crownpreparation
MANDIBULAR PREMOLARMODIFIED THREE-QUARTERCROWNMandibular partial veneer preparations are made more often on premolarsthan on molars. They differ from maxillary molar three-quarter crownpreparations in two respects:Additional retention is required because of the shorter crown lengths ofmandibular teeth. This can be obtained by extending the preparationbuccally, although because of their rather prominent position in the dentalarch, these teeth should be modified only distal to their height of contour.The axial surface that is not prepared (the buccal) includes the functionalcusp. This means that additional tooth structure must be removed toprovide sufficient bulk of metal for strength.
Occlusal reductionPlace 0.8-mm depth grooves on the buccal inclines of the lingual cusp and1.3-mm grooves on the lingual inclines of the buccal cusp.These guiding grooves are once again placed to follow the basic grooveand fissure pattern of the occlusal surface. Only one depth cut needs to beplaced to accommodate the functional cusp bevel on the distal aspect ofthe distal ridge.Reduce the occlusal surface by removing the toothstructure between the grooves.Depth holes Guiding groovesOcclusal reduction
Axial reductionPlace guiding grooves on the lingual surface to parallel the proposed pathof withdrawal and the long axis of the tooth.Prepare the mesial as already described for the three-quarter and seven-eighth crown.Reduce the distal surface as for a complete crown, extending thepreparation to the transitional line angle and onto the buccal surface.However, it should not extend mesially beyond the middle of the distal halfof the buccal surface, and the chamfer should not extend too far cervically;otherwise, the distobuccal line angle will be unnecessarily reduced, whichwould decrease the resistance form.Axial reduction
FinishingThe modified three-quarter crown preparation can include two or threegrooves.Place the mesial and buccal grooves as described for the seven-eighthscrown. Another distal groove may be placed. In general, to gain as muchlength as possible, the grooves of the three-quarter crown should be slightlybuccal. Care must be taken so that the distal groove is slightly closer to thecenter of the distal wall (so the distobuccal line angle will not beundermined).Mesial and buccal groove
Connect the mesial and buccal grooves with a centric cusp chamfer afterthe grooves and mesial flare have been placed and evaluated.The chamfer must be heavy enough to allow 1.5 mm of clearance in thearea of occlusal contact. A regular or thick diamond is used to place thechamfer, which should connect the grooves and provide a protective"staple" linkage of alloy in the completed restoration. Insufficient toothreduction where this chamfer meets the mesial flare is a common error.Finally, all prepared surfaces are smoothed and the internal line anglesrounded.Width of the chamfer on the centric cusp
Maxillary canine three-quarter crown Maxillary central incisor pinledge
The three-quarter crown on a maxillary canine is probably one of the mostdemanding of all tooth preparations. As with such preparations on otherteeth, on a maxillary canine it involves the proximal and lingual surfacesand leaves the facial surface intact. However, the greater degree ofdifficulty stems from the different shape of the canine tooth. Unless theplacement of grooves is determined very precisely in advance, there will bean undesirable display of metal in the interproximal embrasures . Therelatively short proximal walls do not allow much correction after initialgroove placement. Similarly, the greater degree of curvature in eachproximal wall immediately adjacent to the contact area significantlyinfluences the location of the preparations facial marginMAXILLARY CANINE THREE-QUARTER CROWNA. Incisal and lingual reductionB. Axial reduction and groove placementC. Incisal offset and lingual pinhole
Incisal and lingual reductionRemove enough enamel to allow 1 mm of metal thickness. The designof the incisal bevel should prevent contact between opposing teeth andthe incisal margin. However, the original configuration of the facialsurface should be preserved without significant incisal reduction of thetooth. Outlining the anticipated location of the margin with a pencil canbe helpful.Place depth grooves for both the incisal bevel and the lingual reduction.The direction of the bevel may vary some what depending on theconfiguration of the tooth. Generally it will make an angle ofapproximately 45 degrees with the long axis of the tooth.Incisal reduction
After the depth has been verified, perform the reduction. A football or wheelshaped diamond bur is used to reduce the concave lingual wall.The lingual reduction should not extend onto the cingulum itself, which willbe prepared as part of the axial reduction.The lingual reduction should not extend onto the cingulum itself, which willbe prepared as part of the axial reduction.Lingual surface reduction Half lingual surfacereductionWheel shapeddiamond burLingual reduction
Axial reduction and grooveplacementThe path of withdrawal of the restoration must be accurately determinedbefore axial reduction. Mesiodistally it should parallel the long axis of thetooth; buccolingually it should parallel the middle third or incisal two thirdsof the facial surface. This will permit the preparation of proximal grooves ofoptimum length in an area of the tooth where sufficient bulk is present.To enhance the retention and resistance form of the preparation, place aslightly exaggerated chamfer on the lingual aspect of the tooth and aguiding groove in the middle of the lingual wall. When alignment has beenverified, the axial reduction can be performed in the same manner as theother preparations.Axial reduction – mesiodistally the bur is orientedparallel to the long axis of the toothHalf axial reductionRegular grit diamondbur
It is important to understand the difference between this phase of thepreparation on a canine, with little bulk of lingual tooth structure asopposed to a premolar or molar. After completion, a proximal flangeshould result that will guide the rotary instrument during grooveplacement.The technical aspects of the preparation of proximal grooves are like thosedescribed for the other partial veneer preparations. The primary differenceis the direction in which the groove is prepared.Axial reduction Groove FlareAxial reduction Completed axial reduction
Because the groove is placed perpendicular to the proximal wall, its deepestportion will be slightly labial to the proximal flange that results whenproximo-axial reduction is completed. As a result, the proximal flares willextend slightly farther onto the facial surface. This is even more accentuatedby the curvature of the proximal wall. Meticulous assessment of the neededextent of the initial axial reduction is a prerequisite for successfulpreparation.Required interproximalclearanceProximal flare on premolars andcaninesProximalgrooves
Incisal offset and lingualpinholeAnterior partial veneer crowns require a means of reinforcement forpreserving the castings integrity. Posterior three-quarter crowns usually donot need as much additional reinforcement because the solid "corrugated”occlusal surface provides rigidity. For an anterior tooth, an incisal offset orgroove is needed to create a band of thicker metal to provide a "staple"configuration. This provides additional rigidity and resistance againstbending of the casting.Three quarter crown preparation
Connect the mesial and distal grooves with an incisal offset. It shouldimprove the general resistance form of the preparation against lingualdisplacement and should have a V configuration. Sufficient dentin must bepreserved facially to the offset to prevent the metal from being visiblethrough the translucent tooth enamel. This is most effectivelyaccomplished with an offset that is slightly narrower labiolingually thanincisocervically.The offset should follow the normal configuration of the incisal edge, andits transition into the proximal flares should be smooth and continuous. Aninverted-cone diamond or carbide can be used to prepare the offset.Place a pinhole in the cingulum area slightly off center to improve theretention and resistance form of this preparation.Preparation of incisal offset / faciolingual inclination of the rotaryinstrumentsInverted cone carbide bur
The pinhole is prepared in five stages:first, a small horizontal ledge is made with a large, tapered carbide bur;second, a slight "dimple" is created with a round bur at the intended pinholelocation;third, a pilot hole is prepared with a small-diameter twist drill (it must beparallel to the precise path of withdrawal of the restoration);fourth, the preparation is completed with a tapered carbide bur to a pinholedepth of approximately 2 mm;finally, a larger, round bur is used to countersink or bevel the junction betweenpinhole and ledge.
PINLEDGE PREPARATIONA pinledge is occasionally used as a single restoration, generally toreestablish anterior guidance, in which case only the lingual surface isprepared. More commonly, however, it is used as a retainer for a fixedpartial denture or to splint periodontally compromised teeth.In these cases, one or more of the proximal surfaces are included in thepreparation design to accommodate the required connector(s). Retentionand resistance are provided primarily by pins that extend to a depth of 2mm into dentin. Compared to other retainers, the pinledge preparation isvery conservative of tooth structure.The preparation steps themselves are not difficult, but advance planning and athorough understanding of the various steps are prerequisites to success.Diagnostic preparation on an accurate cast is particularly useful during theplanning phase.Four unit FPD with modified pinledge Modified pinledge serving as a retainer for a four unitFPD
Preparation of a number of parallel pinholes with a common path ofwithdrawal can be intimidating. With some practice, however, this can beaccomplished freehand by most operators, especially when a tapered buris used.Paralleling devices are available for practitioners who do not feelcomfortable preparing multiple pinholes. Generally, pinledges are highlyesthetic restorations. Plaque control after treatment is easier because ofshort margin length and largely supragingival margin location.Tapered burPinledge splints
IndicationThe pinledge is indicated for undamaged anterior teeth in dentitions with alow caries experience. The presence of a small proximal carious lesion,however, does not preclude its use.If a high esthetic requirement exists, the advantage of this restoration isthat the labial tooth surface remains intact, although this is sometimesoffset by the display of a slight amount of metal along the incisal edge.Pinledges can be prepared on bulbous teeth that are unsuitable for three-quarter crowns, which would result in a significant amount of unsupportedenamel interproximally.The lingual concavity of a maxillary anterior tooth can be modifiedsuccessfully with a pinledge restoration to establish the desired anteriorguidance.
ContraindicationPatients with poor oral hygiene or a high caries rate are not good candidatesfor this type of restoration. Young patients with large pulps generally arebetter served by a resin-retained FPD.Often it is not possible to place pinholes of adequate size and length in teeththat are thin labiolingually.Pinledges are contraindicated on non-vital teeth and when the alignment ofthe abutment will conflict with the proposed path of withdrawal of the fixedpartial denture. Because less surface area is involved in the preparation,pinledges are not as retentive as their less conservative counterparts.Therefore they should not be used when optimum retention is needed.Pinledge preparation on a maxillary central incisor
MAXILARY CENTRAL INCISORPINLEDGEA. Conventional pinledgeB. Pinledge with proximal sliceC. Pinledge with proximal groove
DesignDraw the outline of the proposed preparation onto the tooth. A line ismarked along the height of contour of the incisal edge and on theproximal wall to include the area needed for a connector. The lingualchamfer is placed immediately adjacent to the crest of the marginalridge. The cervical extent of the margin is on the height of contour of thecingulum, but it may be extended farther cervically at a later stage toblend into the proximal aspect of the preparation.Malpositioned six caries free anterior teeth are for pinledge preparation
Proximal reductionPrepare the proximal slice with a tapered diamond.The diamond is either held parallel to the path of withdrawal or given a slightlingual inclination. The primary purpose of this step is to provide sufficientreduction to allow adequate metal in the area for a subsequent connector.The proximal reduction includes the proximal contact area, but care must betaken not to extend the reduction too far facially, because this will alter theoutline form of the tooth. For esthetic reasons, the reduction must not extendonto the labial surface.
Incisal and lingual reductionPrepare the incisal bevel with the diamond inclined slightly toward thelingual. It extends just beyond the previously placed pencil line on thecrest of the incisal edge, but it must remain within the curvature of theincisal edge to minimize display of metal. Sufficient clearance providesfunctional contact on metal rather than on the junction between metaland tooth structure. The desired metal thickness is 1 mm, except in thearea close to the margin.Perform the lingual reduction with a foot-ball or wheel-shaped diamondafter placing reduction grooves as has been described in other anteriorpreparations. Metal thickness of 1 mm is required in the intercuspalposition and throughout excursive movements.The reduction follows the lingual marginal ridge and continues itschamfer configuration cervically until it runs into the proximal reduction.To facilitate subsequent stages of the preparation, care must be taken tomaintain as much tooth structure as possible in the incisal third.Smooth the incisal and lingual reduction with fine-grit diamonds andstones before preparing the ledges and pinholes
Ledges and indentationsTwo ledges are prepared across the reduced lingual surface. They willprovide room for sufficient bulk of metal to ensure rigidity. Therestoration would otherwise not be very strong because it would consistof only a thin sheet of metal.The ledges are prepared parallel to the incisal edge of the tooth asviewed from the lingual and parallel to one another as viewed from theincisal. In selected areas they will be widened to provide indentations ofsufficient size to accommodate the pinholes. The determination of theinciso-cervical location of the ledges depends on the configuration of thepulp and the available bulk of tooth structure.Location of the ledges relative to the height of the crown
Usually the incisal ledge is prepared 2 to 2.5 mm cervical to the incisaledge, or one fourth of the total height of the preparation from the incisaledge. The cervical ledge is placed on the crest of the cingulum at thecenter of the cervical one fourth of the preparation.Prepare two ledges with a cylindrical carbide bur. The recommendedminimum width for the ledge is 0.7 mm. Drawing the proposed location ofthe ledges on the lingual surface of the tooth is helpful. The design of theledges must be compatible with the path of withdrawal of the restoration,which is parallel to the incisal two thirds of the labial surface of the tooth.Relationship between pinhole placement and pulp configurationMaxillary Central incisor Maxillary Lateral incisor Maxillary canine
Cross section through cervical pinholesCross-section through incisal pinholeMaxillary Central incisor Maxillary Lateral incisor Maxillary canineMaxillary Central incisor Maxillary Lateral incisor Maxillary canine
Make indentations in the left and right sides of the incisal ledge andslightly off center in the cervical ledge to prevent subsequent pulpexposure when the pinholes are placed. These incisal indentations will beas widely spaced as possible to retain as much dentin as possiblebetween the pinholes and the pulp.Because the completed pinhole must be surrounded by sound dentin, it isnot possible to place holes in the extreme corners because of the toothsmorphology. However, every effort should be made to prepare theindentations so that the pinholes will be surrounded by dentin and awayfrom the pulp. This is particularly important for younger patients.Generally this means that the indentations are just within the mesial anddistal marginal ridges, about 1.5 mm inside the external tooth contour. Thesame carbide bur can be used to prepare the indentations.When completed, the configuration of the indentations should resemble ahalf cylinder. Again, their orientation is parallel to the selected path ofwithdrawal and their floor should be smooth and continuous with the floorof the ledges. When combined, they should provide a flat area 1 to 1.2mm wide buccolingually.
Pinhole preparationSink pilot channels with either a small, round bur or a small twist drill. Theshallow indentations will prevent skating of the selected bur. The depth ofthe completed pinhole should be at least 2 mm but can be as much as 3mm when the placement and orientation of the pilot channels aresatisfactory.Enlarge and deepen the pilot channels with a tapered carbide bur whentheir placement and orientation are satisfactory. At this stage, any smallcorrections in orientation can be made. Less experienced operators mayspend a great deal of time attempting to determine the correct alignmentof the bur.However, it should be remembered that the design and location of thepinholes have already been determined by the placement of the ledgesand indentations, so the only remaining concern should be verification ofthe position of the rotary instrument and attainment of the minimum depthof the pinholes.
Some operators find it helpful to place a second bur in a prepared pinholeto help transfer the path of withdrawal, although precautions must betaken to prevent its being swallowed or inhaled. Preparing multiplepinholes a little at a time may also be helpful, moving from one to thenext and gradually deepening each. This will permit alignment verificationas the pinholes areprepared.Bevel the junction between pinhole and indentation with a round burslightly larger than the largest diameter of the pinhole.Inspect all surfaces of the preparation for smoothness and evaluate themargin. Correct any area that requires more distinct delineation.